1. Field of the Invention
The present invention relates to a method of manufacturing a plastic seal semiconductor device and an apparatus for performing the same method.
2. Description of the Related Art
A plastic seal semiconductor device is manufactured by mounting a semiconductor element on an element mounting portion of a lead frame, connecting top ends of metal thin wires to the element, holding the lead frame within a resin mold and injecting plastic material into the mold to seal the semiconductor element mounted on the lead frame, the thin metal wires and internal leads together.
According to a conventional technique, an individual semiconductor device is manufactured by, after aging for stabilizing the plastic material, removing tie-bars and unnecessary plastic portion of the seal, processing external leads by such as plating them with solder, separating the semiconductor device from the lead frame and shaping the external leads to predetermined configuration.
FIG. 1A is a perspective view of mold halves for shaping external leads of a semiconductor device called SOP (Small outline Package) and the semiconductor device before its external leads are shaped and FIG. 1B is a cross sectional side view thereof showing the shaping. Further, FIG. 1C is a front view of the semiconductor device after its external leads are shaped and FIG. 1D is a side view thereof.
In FIGS. 1A to 1D, a punch 7 is lowered to press the external leads 1 down with root portions of the external leads 1 protruding from the plastic seal portion 2 being fixed in between lead pressing faces 6 of an upper mold half 5 and lead pressing faces 4 of a lower mold half 3. With a further downward movement of the punch 7, end portions 11 of the external leads 1 are pressed between a lower face 9 of the punch 7 and a lead end pressing face 8 of the lower mold half 3, resulting in gull wing shape external leads 1. That is, the external lead 1 protruding from a side face of the plastic seal portion 2 is bent down at right angle at its root portion and then bent at right angle at its end portion 11 so that the latter becomes in parallel to a main surface of the plastic seal portion 2. The end portion 11 is bonded to a conductive layer pad (not shown) of a printed circuit board 12.
It has been known, however, that the plastic seal portion 2 may be considerably warped as shown in FIG. 1A. Such warpage may be caused by difference in shrinkage between the lead frame and the sealing plastic material and/or between an upper and a lower plastic material portions, etc.
When the above-mentioned shaping procedure is applied to the external leads 1 of such warped plastic seal portion 2, the warp is temporarily corrected or removed during the shaping process of the external leads 1 shown in FIG. 1B and, therefore, the end portions 11 of the external leads 1 are formed equidistantly from the plastic seal portion 2 since the pressing face 6 of the upper mold half 5 and the pressing face 4 of the lower mold half 3 are flat. However, since the warp is restored at a time when the plastic seal portion 2 is removed from the lead shaping mold halves, the end portions 11 of the external leads 1 having gull wing shape are deviated according to the warp. As a result, the end portions 11 of the external leads 1 are not in a common plane as shown in FIG. 1C and, therefore, there may be unsatisfactory connection between the conductive layer pad formed on the printed circuit board 12 and the end portions 11 when the semiconductor device is actually mounted on the printed circuit board 12. For example, in a case where a lead pitch is 0.5 mm, it is necessary to make a maximum difference in height between end portions of the respective external leads not more then 55 .mu.m. In the conventional case shown in FIG. 1C, however, the difference between the end portions 11 of the outermost external leads 1 and the center external lead 1 becomes larger than 55 .mu.m, causing unacceptable connection between the end portion 11 of the center external lead 1 and the underlying conductive layer pad.
A technique for solving this problem is disclosed in Japanese Patent Application Laid-open No. Hei 2-210855 and FIGS. 2A to 2C show the technique. FIG. 2A is a cross sectional side view of a mold and a portion of external leads which is being molded by the mold, FIG. 2B is a front view of a semiconductor device whose leads are molded and FIG. 2C is a side view of the semiconductor device.
In FIG. 2A corresponding to FIG. 1B, a lead pressing face 26 of an upper mold half 25 and a lead pressing face 24 of a lower mold half 23 which are clamping root portions of external leads 1 are stepped correspondingly to an expected warp of a plastic seal portion. Since a length of a center external lead 1 is made longer vertically than those of outermost external leads 1 due to the stepped configuration of the upper and lower pressing faces 24 and 26 of the mold halves 23 and 25 as shown in FIGS. 2B and 2C, end portions 11 of all of the external leads 1 can be in a common plane, resulting in no such problem as encountered in the conventional case shown in FIG. 1D.
However, warping of a plastic seal portion of a semiconductor device and its direction depends upon individual semiconductor devices and, therefore, it is practically impossible to determine the configuration of the pressing faces of the mold halves applicable to all semiconductor devices.